Somitogenesis consists of the segmentation and differentiation of the paraxial mesoderm into the skeletal muscle, axial skeleton and dermis of the adult. While many genes involved in regulating the periodic nature of somite formation have been characterized, the cell behaviors underlying this process remain poorly understood. The objective of this proposal is to determine the molecular mechanism underlying somite morphogenesis in the vertebrate Xenopus laevis. Our central hypothesis is that the secreted cytokine, stromal-derived factor-1 (SDF-1), regulates Xenopus somite morphogenesis by activating the specific actin regulatory molecules RhoA, Rac1, or Cdc42. Specific support for this hypothesis is based on recent studies in zebrafish that showed that the cell behaviors associated with somite rotation (Hollway et al., 2007;Stellabote et al., 2007) are similar to those documented by our lab and others in Xenopus (Afonin et al., 2006). Furthermore, the secreted cytokine, stromal-derived factor-1 (SDF-1), was shown to be required for the cell rearrangements associated with zebrafish somite rotation (Hollway et al., 2007), which is significant as it is the first signaling molecule shown to regulate somite rotation. In Xenopus, SDF-1 was shown to play a role in mesoderm migration during gastrulation (Fukui et al., 2006) and is expressed in axial tissues throughout early embryonic development (Braun et al., 2002). Thus, SDF-1 is present at the right time and place to play a role in somite morphogenesis. To test our central hypothesis, we will modulate SDF-1 signaling by using morpholinos to knockdown expression levels and in vivo mRNA injections to up-regulate protein levels of SDF-1 and its receptor, CXCR4, in the cell (aim 1). Previous studies have shown that SDF-1 signaling affects changes in cell behaviors by activating specific Rho GTPases (Tan et al., 2006). These actin regulatory molecules have been shown to play an important role in tissue remodeling and thus are likely downstream targets of the SDF-1 signaling pathway during somitogenesis. To test this hypothesis, we will determine whether SDF-1 signaling modulates the activity of specific Rho GTPases during Xenopus somitogenesis (aim 2). Another goal of this proposal is to introduce new molecular approaches that along with the existing expertise in imaging techniques will provide an excellent environment for training students, many of whom are underrepresented minorities (aim 3). Furthermore, we also developed a mentorship plan with three senior scientists (Drs. Amacher, Symes, and Weisblat) whose research expertise complements the objectives presented in this proposal and will increase the research productivity of our lab. Relevance to Public Health: The proposed research is expected to provide critical information on how vertebrate somites are formed. This information will help in developing tools for diagnosing or preventing vertebral disorders such as scoliosis and other congenital spinal cord deformities.